The vascular endothelium lines the inside of all blood vessels. It acts as the interface between the blood and the tissues and organs. The endothelium forms a semi-permeable barrier that maintains the integrity of the blood fluid compartment, but permits passage of water, ions, small molecules, macromolecules and cells in a regulated manner. Dysregulation of this process produces vascular leakage into underlying tissues. Leakage of fluid into tissues causing edema can have serious and life threatening consequences in a variety of diseases. Accordingly, it would be highly desirable to have a method for reducing edema, preferably at its earliest stage, and restoring the endothelial barrier to physiological.
The endothelium is a key gatekeeper controlling the exchange of molecules from the blood to the tissue parenchyma. It largely controls the permeability of a particular vascular bed to blood-borne molecules. The permeability and selectivity of the endothelial cell barrier is strongly dependent on the structure and type of endothelium lining the microvasculature in different vascular beds. Endothelial cells lining the microvascular beds of different organs exhibit structural differentiation that can be grouped into three primary morphologic categories: sinusoidal, fenestrated and continuous.
Sinusoidal endothelium (also referred to as “discontinuous endothelium”) has large intercellular and intracellular gaps and no basement membrane, allowing for minimally restricted transport of molecules from the capillary lumen into the tissue and vice versa. Sinusoidal endothelium is found in liver, spleen and bone marrow.
Fenestrated endothelia are characterized by the presence of a large number of circular transcellular openings called fenestrae with a diameter of 60 to 80 nm. Fenestrated endothelia are found in tissues and organs that require rapid exchange of small molecules, including kidney (glomeruli, peritubular capillaries and ascending vasa recta), pancreas, adrenal glands, endocrine glands and intestine. The fenestrae are covered by thin diaphragms, except for those in mature, healthy glomeruli. See Ichimura et al., J. Am. Soc. Nephrol., 19:1463-1471 (2008).
Continuous endothelia do not contain fenestrae or large gaps. Instead, continuous endothelia are characterized by an uninterrupted endothelial cell monolayer. Most endothelia in the body are continuous endothelia, and continuous endothelium is found in, or around, the brain (blood brain barrier), diaphragm, duodenal musculature, fat, heart, some areas of the kidneys (papillary microvasculature, descending vasa recta), large blood vessels, lungs, mesentery, nerves, retina (blood retinal barrier), skeletal muscle, testis and other tissues and organs of the body.
Endothelial transport in continuous endothelium can be thought of in a general sense as occurring by paracellular and transcellular pathways. The paracellular pathway is the pathway between endothelial cells, through the interendothelial junctions (IEJs). In unperturbed continuous endothelium, water, ions and small molecules are transported paracellularly by diffusion and convection. A significant amount of water (up to 40%) also crosses the endothelial cell barrier transcellularly through water-transporting membrane channels called aquaporins. A variety of stimuli can disrupt the organization of the IEJs, thereby opening gaps in the endothelial barrier. The formation of these intercellular gaps allows passage of fluid, ions, macromolecules (e.g., proteins) and other plasma constituents between the endothelial cells in an unrestricted manner. This paracellular-caused hyperpermeability produces edema and other adverse effects that can eventually result in damage to tissues and organs.
The transcellular pathway is responsible for the active transport of macromolecules, such as albumin and other plasma proteins, across the endothelial cells, a process referred to as “transcytosis.” The transport of macromolecules occurs in vesicles called caveolae. Almost all continuous endothelia have abundant caveolae, except for continuous endothelia located in brain and testes which have few caveolae. Transcytosis is a multi-step process that involves successive caveolae budding and fission from the plasmalemma and translocation across the cell, followed by docking and fusion with the opposite plasmalemma, where the caveolae release their contents by exocytosis into the interstitium. Transcytosis is selective and tightly regulated under normal physiological conditions.
There is a growing realization of the fundamental importance of the transcellular pathway. Transcytosis of plasma proteins, especially albumin which represents 65% of plasma protein, is of particular interest because of its ability to regulate the transvascular oncotic pressure gradient. As can be appreciated, then, increased transcytosis of albumin and other plasma proteins above basal levels will increase the tissue protein concentration of them which, in turn, will cause water to move across the endothelial barrier, thereby producing edema.
Low density lipoproteins (LDL) are also transported across endothelial cells by transcytosis. In hyperlipidemia, a significant increase in transcytosis of LDL has been detected as the initial event in atherogenesis. The LDL accumulates in the subendothelial space, trapped within the expanded basal lamina and extracellular matrix. The subendothelial lipoprotein accumulation in hyperlipidema is followed by a cascade of events resulting in atheromatous plaque formation. Advanced atherosclerotic lesions are reported to be occasionally accompanied by the opening of IEJs and massive uncontrolled passage of LDL and albumin.
Vascular complications are a hallmark of diabetes. At the level of large vessels, the disease appears to be expressed as an acceleration of an atherosclerotic process. With respect to microangiopathy, alterations in the microvasculature of the retina, renal glomerulus and nerves cause the greatest number of clinical complications, but a continuously increasing number of investigations show that diabetes also affects the microvasculature of other organs, such as the mesentery, skin, skeletal muscle, heart, brain and lung, causing additional clinical complications. In all of these vascular beds, changes in vascular permeability appear to represent a hallmark of the diabetic endothelial dysfunction.
In continuous endothelium, capillary hyperpermeability to plasma macromolecules in the early phase of diabetes is explained by an intensification of transendothelial vesicular transport (i.e., by increased transcytosis) and not by the destabilization of the IEJs. In addition, the endothelial cells of diabetics, including those of the brain, have been reported to contain an increased number of caveolae as compared to normals, and glycated proteins, particularly glycated albumin, are taken up by endothelial cells and transcytosed at substantially greater rates than their native forms. Further, increased transcytosis of macromolecules is a process that continues beyond the early phase of diabetes and appears to be a cause of edema in diabetic tissues and organs throughout the disease if left untreated. This edema, in turn, leads to tissue and organ damage. Similar increases in transcellular transport of macromolecules have been reported in hypertension.
Paracellular-caused hyperpermeability is also a factor in diabetes and the vascular complications of diabetes. The IEJs of the paracellular pathway include the adherens junctions (AJs) and tight junctions (TJs). Diabetes alters the content, phosphorylation and localization of certain proteins in both the AJs and TJs, thereby contributing to increased endothelial barrier permeability.
In support of the foregoing discussion and for further information, see Frank et al., Cell Tissue Res., 335:41-47 (2009), Simionescu et al., Cell Tissue Res., 335:27-40 (2009); van den Berg et al., J. Cyst. Fibros., 7(6): 515-519 (2008); Viazzi et al., Hypertens. Res., 31:873-879 (2008); Antonetti et al., Chapter 14, pages 340-342, in Diabetic Retinopathy (edited by Elia J. Duh, Humana Press, 2008), Felinski et al., Current Eye Research, 30:949-957 (2005), Pascariu et al., Journal of Histochemistry & Cytochemistry, 52(1):65-76 (2004); Bouchard et al., Diabetologia, 45:1017-1025 (2002); Arshi et al., Laboratory Investigation, 80(8):1171-1184 (2000); Vinores et al., Documenta Ophthalmologica, 97:217-228 (1999); Oomen et al., European Journal of Clinical Investigation, 29:1035-1040 (1999); Vinores et al., Pathol. Res. Pract., 194:497-505 (1998); Antonetti et al., Diabetes, 47:1953-1959 (1998), Popov et al., Acta Diabetol., 34:285-293 (1997); Yamaji et al., Circulation Research, 72:947-957 (1993); Vinores et al., Histochemical Journal, 25:648-663 (1993); Beals et al., Microvascular Research, 45:11-19 (1993); Caldwell et al., Investigative Ophthalmol. Visual Sci., 33(5):16101619 (1992).
Endothelial transport in fenestrated endothelium also occurs by transcytosis and the paracellular pathway. In addition, endothelial transport occurs by means of the fenestrae. Fenestrated endothelia show a remarkably high permeability to water and small hydrophilic solutes due to the presence of the fenestrae.
The fenestrae may or may not be covered by a diaphragm. The locations of endothelium with diaphragmed fenestrae include endocrine tissue (e.g., pancreatic islets and adrenal cortex), gastrointestinal mucosa and renal peritubular capillaries. The permeability to plasma proteins of fenestrated endothelium with diaphragmed fenestrae does not exceed that of continuous endothelium.
The locations of endothelium with nondiaphragmed fenestrae include the glomeruli of the kidneys. The glomerular fenestrated endothelium is covered by a glycocalyx that extends into the fenestrae (forming so-called “seive plugs”) and by a more loosely associated endothelial cell surface layer of glycoproteins. Mathematical analyses of functional permselectivity studies have concluded that the glomerular endothelial cell glycocalyx, including that present in the fenestrae, and its associated surface layer account for the retention of up to 95% of plasma proteins within the circulation.
Loss of fenestrae in the glomerular endothelium has been found to be associated with proteinuria in several diseases, including diabetic nephropathy, transplant glomerulopathy, pre-eclampsia, diabetes, renal failure, cyclosporine nephropathy, serum sickness nephritis and Thy-1 nephritis. Actin rearrangement and, in particular, depolymerization of stress fibers have been found to be important for the formation and maintenance of fenestrae.
In support of the foregoing discussion of fenestrated endothelia and for additional information, see Satchell et al., Am. J. Physiol. Renal Physiol., 296:F947-F956 (2009); Haraldsson et al., Curr. Opin. Nephrol. Hypertens., 18:331-335 (2009); Ichimura et al., J. Am. Soc. Nephrol., 19:1463-1471 (2008); Ballermann, Nephron Physiol., 106:19-25 (2007); Toyoda et al., Diabetes, 56:2155-2160 (2007); Stan, “Endothelial Structures Involved In Vascular Permeability,” pages 679-688, Endothelial Biomedicine (ed. Aird, Cambridge University Press, Cambridge, 2007); Simionescu and Antohe, “Functional Ultrastructure of the Vascular Endothelium: Changes in Various Pathologies,” pages 42-69, The Vascular Endothelium I (eds. Moncada and Higgs, Springer-Verlag, Berlin, 2006).
Endothelial transport in sinusoidal endothelium occurs by transcytosis and through the intercellular gaps (interendothelial slits) and intracellular gaps (fenestrae). Treatment of sinusoidal endothelium with actin filament-disrupting drugs can induce a substantial and rapid increase in the number of gaps, indicating regulation of the porosity of the endothelial lining by the actin cytoskeleton. Other cytoskeleton altering drugs have been reported to change the diameters of fenestrae. Therefore, the fenestrae-associated cytoskeleton probably controls the important function of endothelial filtration in sinusodial endotheluium. In liver, defenestration (loss of fenestrae), which causes a reduction in permeability of the endothelium, has been associated with the pathogenesis of several diseases and conditions, including aging, atherogenesis, atherosclerosis, cirrhosis, fibrosis, liver failure and primary and metastatic liver cancers. In support of the foregoing and for additional information, see Yokomori, Med. Mol. Morphol., 41:1-4 (2008); Stan, “Endothelial Structures Involved In Vascular Permeability,” pages 679-688, Endothelial Biomedicine (ed. Aird, Cambridge University Press, Cambridge, 2007); DeLeve, “The Hepatic Sinusoidal Endothelial Cell,” pages 1226-1238, Endothelial Biomedicine (ed. Aird, Cambridge University Press, Cambridge, 2007); Pries and Kuebler, “Normal Endothelium,” pages 1-40, The Vascular Endothelium I (eds. Moncada and Higgs, Springer-Verlag, Berlin, 2006); Simionescu and Antohe, “Functional Ultrastructure of the Vascular Endothelium: Changes in Various Pathologies,” pages 42-69, The Vascular Endothelium I (eds. Moncada and Higgs, Springer-Verlag, Berlin, 2006); Braet and Wisse, Comparative Hepatology, 1:1-17 (2002); Kanai et al., Anat. Rec., 244:175-181 (1996); Kempka et al., Exp. Cell Res., 176:38-48 (1988); Kishimoto et al., Am. J. Anat., 178:241-249 (1987).
Diabetic retinopathy is the most common diabetic eye disease and a leading cause of blindness in American adults (National Eye Institute factsheet, 2009 at www.nei.nih.gov/health/diabetic/retinopathy.asp). In 2000, the World Health Organization published that the prevalence of diabetes in the United States was reported to be 17,702,000. The report also stated that the prevalence is expected to rise to 30,312,000 by 2030 (Wild, 2004). Diabetic retinopathy is a progressive and cumulative change in the retinal vasculature that includes microaneurysms, intra-retinal hemorrhage and exudate, vascular tortuosity, intra-retinal microvascular anomalies (IRMA) and pre-retinal neovascularization (Boyd, et al. Canadian Diabetes Association Clinical Practice Guidelines Expert Committee 2008, S134-139). Pre-retinal neovascularization can lead to vitreous hemorrhage, retinal detachment, fibrosis and permanent vision loss. Diabetic retinopathy also includes Diabetic Macular Edema (DME), which is the extravasation of fluid that involves or threatens central vision. Most visual loss in diabetes is due to DME (Moss, et al., Ophthalmology 1998; 105:998-1003). Increased vascular permeability and edema occur at an early stage in this process. Effective treatment of vascular permeability and edema may reverse or slow these complications of diabetes before retinal tissue is permanently damaged (Gardner, et al. Current Diabetes Reports 2008; 8:263-269; Sander, et al. Invest Ophthalmol Vis. Sci. 2007; 48:3983-3987; and Antonetti, et al. Diabetes 2006; 55:2401-2411).
The primary treatments for clinically significant diabetic macular edema (CSME) consist of retinal laser photocoagulation and intravitreal ranibizumab. Retinal laser photocoagulation to areas of leakage can reduce moderate visual loss by 50% (from 30% to 15%) and slow the progression of disease. However, laser treatments are limited by a lack of efficacy in some cases, procedural discomfort, the need for repeated treatments, and a risk of ablative retinal damage, including foveal burns and scars that may increase over time (American Academy of Ophthalmology Preferred Practice Pattern®, Diabetic Retinopathy, 2008; Maeshima, et al. Retina. 2004; 24:507-511). Intravitreal ranibizumab (LUCENTIS®), was approved for the treatment of DME on Jul. 30, 2012. This anti-vascular endothelial growth factor (VEGF) therapy, an injection to the eye, was shown to be effective but cannot be administered to some patients for a number of reasons: risk for immunological reaction, glaucoma, local eye irritation or development of endopthalmitis which may lead to complete vision loss. Additionally, high treatment cost for a procedure that is repeated monthly makes it difficult for many patients to afford.
There is no effective oral drug treatment for diabetic retinopathy, specifically DME, other than general measures such as controlling blood sugar, hypertension and blood lipids. A significant unmet clinical need exists for novel drug therapies that can effectively treat diabetic retinopathy and DME (Ryan, et al. Am. J. Health Syst Pharm. 2007; 64(17Suppl, 12):S15-21).